Immune activation correlates better than HIV plasma viral load with CD4 T-cell decline during HIV infection

Feline Immunodeficiency Virus Infection Phenotypically and Functionally Activates Immunosuppressive CD4+CD25+T Regulatory Cells

Disease progression of feline immunodeficiency virus (FIV) infection is characterized by up-regulation of B7.1 and B7.2 costimulatory molecules and their ligand CTLA4 on CD4(+) and CD8(+) T cells. The CD4(+)CTLA4(+)B7(+) phenotype described in FIV(+) cats is reminiscent of CD4(+)CD25(+)CTLA4(+) cells, a phenotype described for immunosuppressive T regulatory (Treg) cells. In the present study, we describe the phenotypic and functional characteristics of CD4(+)CD25(+) T cells in PBMC and lymph nodes (LN) of FIV(+) and control cats. Similar to Treg cells, feline CD4(+)CD25(+) but not CD4(+)CD25(-) T cells directly isolated from LN of FIV(+) cats do not produce IL-2 and fail to proliferate in response to mitogen stimulation. Unstimulated CD4(+)CD25(+) T cells from FIV(+) cats significantly suppress the proliferative response and the IL-2 production of Con A-stimulated autologous CD4(+)CD25(-) T cells compared with unstimulated CD4(+)CD25(+) T cells from FIV(-) cats. Flow-cytometric analysis confirmed the apparent activation phenotype of the CD4(+)CD25(+) cells in LN of chronically FIV(+) cats, because these cells showed significant up-regulation of expression of costimulatory molecules B7.1, B7.2, and CTLA4. These FIV-activated, anergic, immunosuppressive CD25(+)CTLA4(+)B7(+)CD4(+) Treg-like cells may contribute to the progressive loss of T cell immune function that is characteristic of FIV infection.

Increased Proportions of Activated and Proliferating Memory CD8+ T Lymphocytes in Both Blood and Lung are Associated with Blood HIV Viral Load

One mechanism of HIV pathogenesis has been proposed to be the activation of T lymphocytes, resulting in proliferation and decreased survival of these activated cells. Others have suggested that co-infections may exacerbate this process. These hypotheses were tested by examining the relationship between HIV blood viral load with the proportion of activated and proliferating CD8+ lymphocytes in lung and blood. In the lung these responses were compared in patients with or without respiratory pathogens. Thirty-five HIV-positive patients and five controls underwent bronchoalveolar lavage (BAL). BAL and blood samples were fixed and permeabilized and the proportions of memory CD8+ lymphocytes that expressed the activation marker CD38 and the cell cycling marker Ki67 were measured by flow cytometry. CD38bright CD8+ lymphocytes from both sites increased with increasing blood viral load. In BAL there was no significant difference between the CD38 activation status in those with respiratory pathogens compared with those without. More CD38bright CD8+ lymphocytes expressed Ki67 when compared with the CD38dim lymphocytes. These findings provide evidence that HIV is the primary factor in stimulating CD8+ cell activation.

T cell dynamics in HIV-1 infection

In the absence of antiretroviral treatment, HIV-1 establishes a chronic, progressive infection of the human immune system that invariably, over the course of years, leads to its destruction and fatal immunodeficiency. Paradoxically, while viral replication is extensive throughout the course of infection, deterioration of conventional measures of immunity is slow, including the characteristic loss of CD4(+) T cells that is thought to play a key role in the development of immunodeficiency. This conundrum suggests that CD4(+) T cell-directed viral cytopathicity alone cannot explain the course of disease. Indeed, recent advances now indicate that HIV-1 pathogenesis is likely to result from a complex interplay between the virus and the immune system, particularly the mechanisms responsible for T cell homeostasis and regeneration. We review these data and present a model of HIV-1 pathogenesis in which the protracted loss of CD4(+) T cells results from early viral destruction of selected memory T cell populations, followed by a combination of profound increases in overall memory T cell turnover, damage to the thymus and other lymphoid tissues, and physiological limitations in peripheral CD4(+) T cell renewal.

Correlates of immune activation marker changes in human immunodeficiency virus (HIV)-seropositive and high-risk HIV-seronegative women who use illicit drugs

The majority of natural history studies of human immunodeficiency virus (HIV) infection have immune and viral parameters in men. Data demonstrating that women have lower HIV-1 RNA levels than men at the same CD4 cell counts have raised the question of immunologic differences in HIV-seropositive women. This study describes levels and changes in phenotypic markers of immune maturity, function, and activation in the CD4 and CD8 cell subsets in HIV-seropositive and high-risk HIV-seronegative women. Our primary hypothesis was that activation levels would be significantly higher among illicit drug users. However, results showed that HIV-1 RNA level was the strongest predictor of marker level and that both HIV-1 RNA level and CD4 cell count were independently associated with CD4 activation, but illicit drug use was not. In summary, this study demonstrated that immune activation was a significant pathogenic feature in women and that activation was driven by HIV infection and not illicit drug use.

Incomplete CD4 T cell recovery in HIV-1 infection after 12 months of highly active antiretroviral therapy is associated with ongoing increased CD4 T cell activation and turnover

To evaluate the relationship between T cell turnover, immune activation, and CD4 recovery in HIV infection, 32 antiretroviral-naive HIV-1-infected patients were studied before and after initiation of highly active antiretroviral therapy (HAART). Elevated CD4 and CD8 T cell turnover (measured by Ki67) in HIV infection decreased with HAART in blood and lymphoid tissue. Increased peripheral CD4 T cell turnover was strongly associated with immune activation even after viral suppression to less than 50 copies/mL (R = 0.8; p <.001). Increased CD4 T cell turnover correlated strongly with CD4 cell counts both before (R = -0.6; p <.001) and after (R = -0.4; p =.05) HAART. In patients with baseline CD4 cell counts of less than 350/microL, decreases in CD4 T cell turnover with HAART significantly correlated with increases in CD4 cell counts. In addition, persistently elevated levels of CD4 T cell turnover after HAART were associated with incomplete CD4 T cell recovery despite HIV RNA levels of less than 50 copies/mL. These data suggest that immune activation is central to CD4 cell depletion in HIV infection and immune reconstitution with HAART.

Turnover rates of B cells, T cells, and NK cells in simian immunodeficiency virus-infected and uninfected rhesus macaques

We determined average cellular turnover rates by fitting mathematical models to 5-bromo-2'-deoxyuridine measurements in SIV-infected and uninfected rhesus macaques. The daily turnover rates of CD4(+) T cells, CD4(-) T cells, CD20(+) B cells, and CD16(+) NK cells in normal uninfected rhesus macaques were 1, 1, 2, and 2%, respectively. Daily turnover rates of CD45RA(-) memory T cells were 1%, and those of CD45RA(+) naive T cells were 0.5% for CD4(+) T cells and approximately 1% for CD4(-)CD45RA(+) T cells. In SIV-infected monkeys with high viral loads, the turnover rates of T cells were increased approximately 2-fold, and that of memory T cells approximately 3-fold. The turnover of CD4(+)CD45RA(+) naive T cells was increased 2-fold, whereas that of CD4(-)CD45RA(+) naive T cells was marginally increased. B cells and NK cells also had increased turnover in SIV-infected macaques, averaging 3 and 2.5% per day, respectively. For all cell types studied here the daily turnover rate increased with the decrease of the CD4 count that accompanied SIV infection. As a consequence, the turnover rates of CD4(+) T cells, CD4(-) T cells, B cells, and NK cells within each monkey are strongly correlated. This suggests that the cellular turnover of different lymphocyte populations is governed by a similar process which one could summarize as "generalized immune activation." Because the viral load and the CD4 T cell count are negatively correlated we cannot determine which of the two plays the most important role in this generalized immune activation.

Target cells in vaginal HIV transmission

Understanding the mechanisms of HIV transmission to women will be crucial to the development of effective strategies to curb this epidemic. Current data suggest that HIV has at least two routes to penetrate the vaginal epithelium and reach lymphoid tissues, trans-epithelial migration of infected Langerhans cells or virus penetration into the lamina propria through loss of epithelial integrity resulting in direct infection of lymphocytes, dendritic cells and macrophages.

In vivo dynamics of T cell activation, proliferation, and death in HIV-1 infection: why are CD4+ but not CD8+ T cells depleted?

Deuterated glucose labeling was used to measure the in vivo turnover of T lymphocytes. A realistic T cell kinetic model, with populations of resting and activated T cells, was fitted to d-glucose labeling data from healthy and HIV-1-infected individuals before and after antiretroviral treatment. Our analysis highlights why HIV-1 infection, which increases the fraction of both CD4(+) and CD8(+) T lymphocytes that are proliferating (Ki67(+)), leads to CD4 but not CD8 depletion. We find that HIV-1 infection tends to increase the rates of death and proliferation of activated CD4(+) T cells, and to increase the rate at which resting CD4 T cells become activated, but does not increase the fraction of activated CD4(+) T cells, consistent with their preferential loss in HIV-1-infected individuals. In contrast, HIV-1 infection does not lead to an increase in proliferation or death rates of activated CD8(+) T cells, but did increase the fraction of activated CD8(+) T cells, consistent with these cells remaining in an activated state longer and undergoing more rounds of proliferation than CD4(+) T cells. Our results also explain why telomeres shorten in CD8(+) cells, but not in CD4(+) cells of HIV-1-infected patients, compared with age-matched controls.

Attenuated signaling associated with immune activation in HIV-1-infected individuals

Chronic immune activation is associated with impaired signal transduction. Since such activation is commonly found during HIV-1 infection, we studied cellular responses to non-specific T-cell receptor stimulation of PBMC obtained from 20 HIV-1 non-infected individuals and 23 highly or partially immune activated HIV-1 infected individuals. PBMC proliferation and ERK-1/2 phosphorylation following anti-CD3 stimulation, and constitutive levels of Cbl-b, were determined. Increased levels of Cbl-b, decreased proliferation, and lower ERK-1/2 phosphorylation were found in PBMC of highly immune activated HIV-1 infected individuals. The elevated expression of Cbl-b and impaired phosphorylation of ERK-1/2 associated with immune activation probably contribute to the attenuated proliferative and cellular responses characteristic of HIV-1 infection. Therefore, targeting immune negative modulators, such as Cbl-b, may serve as a novel approach for controlling HIV-1 disease progression.

CD4 T cell depletion is linked directly to immune activation in the pathogenesis of HIV-1 and HIV-2 but only indirectly to the viral load

The causal relationships among CD4 cell depletion, HIV replication, and immune activation are not well understood. HIV-2 infection, "nature's experiment" with inherently attenuated HIV disease, provides additional insights into this issue. We report the finding that in HIV-2 and HIV-1 patients with a comparable degree of CD4 depletion the imbalance in the relative sizes of the naive and memory T cell populations and the up-regulation of CD4 and CD8 cell activation markers (HLA-DR, CD38, CD69, Fas molecules) are similar, even though the viral load in the plasma of HIV-2-infected patients is two orders of magnitude lower than in HIV-1 patients and HIV-2 patients are known to have slower rates of CD4 T cell decline and a better clinical prognosis. Moreover, we found a similar increase in the frequency of cycling CD4 T cells (Ki67+), which was in strong correlation with the expression of activation markers. Finally, the level of T cell anergy, as assessed by the proliferative responses to CD3 stimulation and to a panel of microbial Ags, proved to be comparable in HIV-1 and HIV-2 patients with a similar degree of CD4 depletion despite large differences in viral load. Our data are consistent with a direct causal relationship between immune activation and CD4 cell depletion in HIV disease and an only indirect relation of these parameters to the virus replication rate. Invoking the concept of proximal immune activation and virus transmission, which links efficient transmission of virus to local cell activation and proliferation in response to Ags and inflammation, we propose an integrative interpretation of the data and suggest that strongly elevated immune activation induces CD4 cell depletion and not vice versa, with potential implications for the choice of treatment strategies.

Human retroviruses after 20 years: a perspective from the past and prospects for their future control

Among viruses the human retroviruses may be of special interest to immunologists, because they target cells of the immune system, particularly mature CD4+ T cells, impair their function and cause them to grow abnormally (human T-cell leukemia virus, HTLV) or to die (human immunodeficiency virus, HIV). Human retroviruses cause disease ranging from neurological disorders and leukemias (HTLV-1) to AIDS (acquired immunodeficiency virus) (HIV) and promote development of several types of malignancies (HIV). They share many common features, but their contrasts are greater, especially the far greater replication and variation of HIV associated with its greater genomic complexity. Both have evolved striking redundancy for mechanisms which promote their survival. Thus, HTLV has redundant mechanisms for promoting growth of provirus containing T cells needed for virus continuity, because it is chiefly through its cellular DNA provirus that HTLV replicates and not through production of virions. Conversely, HIV has redundancy in its mechanisms for promoting virion replication and escape from the host immune system. It is via these redundant mechanisms that they produce disease: leukemias from mechanisms promoting T-cell proliferation (HTLV-1) and AIDS from mechanisms promoting virus replication and T-cell death (HIV). The practical challenges for the future are clear. For HTLV-1, education and control of breastfeeding. For HIV, the formidable tasks now ahead in part demand new kinds of talent, talents that will foster greater insights into the development of therapy for the developing countries, new forms of less toxic therapies for all infected persons, a continued and expanded commitment to education, and a persistent 'never say die' commitment to the development of a truly preventive vaccine with all the scientific and nonscientific challenges that these objectives face.

Plasma levels of soluble CD27: a simple marker to monitor immune activation during potent antiretroviral therapy in HIV-1-infected subjects

Plasma levels of soluble CD27 (sCD27) are elevated in diseases characterized by T cell activation and are used as a marker of immune activation. We assessed the usefulness of determining plasma sCD27 as a marker for monitoring immune activation in HIV-1-infected patients treated with highly active antiretroviral therapy (HAART). A first cross-sectional examination of 68 HIV-1-infected and 18 normal subjects showed high levels of sCD27 in HIV-1 infection; plasma sCD27 was correlated to HIV-1 viraemia and inversely correlated to CD4+ T cell count. Twenty-six HIV-1-infected patients undergoing HAART were studied at baseline and after 6, 12, 18 and 24 months of therapy. Seven additional patients under HAART were analysed at baseline, during and after interruption of therapy. In the total population, HAART induced a significant and progressive reduction, but not a normalization, of plasma levels of sCD27 after 24 months. A full normalization of plasma sCD27 was observed in the virological responders (undetectable HIV-1 RNA at months 18 and 24) and also in patients with moderate immunodeficiency at baseline (CD4+ T cell count >200 cells/mm3). Changes in plasma neopterin paralleled the changes in sCD27 but only baseline sCD27 levels were predictive of a greater increase in CD4+ T cell count during the follow-up. Discontinuation of therapy resulted in a rapid increase of sCD27 plasma levels associated with viraemia rebound and drop in CD4+ T cell count. Our findings suggest that plasma sCD27 may represent an alternative and simple marker to monitor immune activation during potent antiretroviral therapy. HIV-1-induced immune activation can be normalized by HAART in successfully treated patients where the disease is not advanced.